首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   31529篇
  免费   341篇
  国内免费   229篇
安全科学   884篇
废物处理   1188篇
环保管理   4399篇
综合类   6146篇
基础理论   8411篇
环境理论   14篇
污染及防治   7910篇
评价与监测   1801篇
社会与环境   1156篇
灾害及防治   190篇
  2018年   334篇
  2017年   354篇
  2016年   560篇
  2015年   439篇
  2014年   637篇
  2013年   2419篇
  2012年   843篇
  2011年   1209篇
  2010年   937篇
  2009年   1007篇
  2008年   1207篇
  2007年   1269篇
  2006年   1156篇
  2005年   938篇
  2004年   924篇
  2003年   947篇
  2002年   884篇
  2001年   1124篇
  2000年   826篇
  1999年   497篇
  1998年   384篇
  1997年   404篇
  1996年   424篇
  1995年   477篇
  1994年   458篇
  1993年   424篇
  1992年   419篇
  1991年   426篇
  1990年   447篇
  1989年   431篇
  1988年   385篇
  1987年   366篇
  1986年   342篇
  1985年   332篇
  1984年   380篇
  1983年   352篇
  1982年   397篇
  1981年   353篇
  1980年   308篇
  1979年   314篇
  1978年   303篇
  1977年   260篇
  1976年   246篇
  1975年   232篇
  1974年   278篇
  1973年   264篇
  1972年   249篇
  1971年   221篇
  1970年   196篇
  1967年   196篇
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
771.
772.
773.
Concern for the acceleration in threats to species diversity in the United States led to the passage of the Endangered Species Act of 1973. In this paper, this legislation is examined in an economic context. A simple model is developed where management and constraints on economic activity are substitutes in the production of species stocks. Whether economic growth constraints (the primary policy tool of the 1973 Act) are justified for preservation purposes depends on technical substitution possibilities, relative cost ratios, and the existence of budget constraints for species management. Since the same combination of these factors will not occur for all species, it is argued that more flexibility in endangered species policy may preserve the same levels of species stocks at lower costs to society.  相似文献   
774.
The exploration of offshore areas for hydrocarbons has involved new technical and economic considerations and has posed new problems, not the least of which has been the changing relationship between companies and governments. In this article, the authors present an analysis of the different perspectives and policies which emanate from national governments on the one hand and from transnational oil companies on the other. The issues examined include the considerations involved in establishing exploration legislation, post-discovery problems, the impact on revenues of increased production, the interests of companies in developing fields and the overall economic environment within which decisions have to be agreed.  相似文献   
775.
Epidemiological procedures can be organised under disaster conditions by means of a simple surveillance system and with few personnel.
In the aftermath of the 4 February 1976 earthquake in Guatemala, an information system was organized by which the requisite information for decision-making was obtained with adequate speed and promptness.
The initial epidemiologic informution was based on reports collected during the early days on symptoms observed at hospitals and health centers and in localities and villages in the stricken area.
At a second, post-emergency stage a more elaborate surveillance system was instituted to provide guidance in the investigntwn of outbreaks, evaluate the health activities and establish basic criteria for preventive and control measures.  相似文献   
776.
777.
778.
The development of traceability in low-level radioactivity measurements is discussed. The role that the development of large quantities of natural matrix standards can play is also discussed.  相似文献   
779.
Although oil spill cleanup requirements have existed in the United States for years, recent increases in oil imports and marine transportation of petroleum products as well as growing environmental concern have exposed a new industry, the Oil Spill Cleanup Industry. This paper explores some of the microeconomic aspects of this industry which has come under increased scrutiny by the general public, big business, and the federal government.In addition to a brief history and definition, several basic questions about the economic viability of the oil spill cleanup industry are raised and explored, and the impact on the industry of cleanup from government sources is examined, both from the perspective of present operations and from apparent future increases in federal participation. The primary dilemma facing the industry, that of providing continued and immediate supply while confronted with stochastic demand, is discussed. The effects of the large spill on the industry both in terms of revenue and ability to meet cleanup requirements is also considered.Information for the paper is drawn from past and continuing involvement in the U.S. Coast Guard's Marine Environmental Protection Program by both authors. The statistical evidence presented here was compiled through personal interviews and from two computerized Coast Guard information systems; PIRS (the Pollution Incident Reporting System), and SKIM (the Spill Cleanup Equipment Inventory System).  相似文献   
780.
The “filter model” has been developed to explain the biologic effects of radiation and chemicals. We have examined nearly 300 sets of dose response data, of which 50 are presented here. Responses (induced by radiation and chemicals) which have been examined include in vitro survival studies on animal and plant tissues, induction of cellular aberrations and time to tumor or death. Similar data from in vivo studies has also been examined. All of the data appear to fit the model R = a lnD + b(lnD)2 + c, where R is the response, a and b are parameters fitted by regression to a particular set of data, and c is the response at zero (or lowest) dose. By writing this model in exponential form, it can be seen that the response R results from multistage filtering (by net amounts a and b) of the initial dose, D. The threshold is obtained from this model as the point, D?T, at which the second derivative becomes zero. This is given by D?T = exp(1 ? a2b) when a and b are oppositelt signed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号